Lignocellulosic biomass represents an attractive and environmentally friendly starting material for production of carbohydrates (such as sugars) and downstream processes (such as fermentation to alcohols, organic acids, polymer precursors, etc.), and/or of lignin, since the raw material is obtained from renewable resources. Many non-food lignocellulosic materials are potential sources, including wood and by-products of wood processing (e.g., chips, sawdust, shavings).
Example methods of producing sugars from lignocellulosic biomass include acid hydrolysis and enzymatic hydrolysis. In acid hydrolysis, a mixture of woody material (such as wood chips) in acid is heated in the presence of water under conditions sufficient to hydrolyze cellulose, producing sugars and lignin residue. US20120264873 of Eyal, et al., discloses an example of a strong hydrochloric acid hydrolysis method. Acid is not consumed in the reaction, so the hydrolysis co-products typically include residual acid content. Additionally, in some acid hydrolysis methods, toxic degradation products are produced, including inhibitors of downstream fermentation (e.g., furans, such as hydroxymethylfurfural (HMF), furfuraldehyde, etc.). Removal of residual acid and inhibitors, such as by waching, ion exchanging, or other purification methods, represents additional cost and time.
Enzymatic hydrolysis proceeds by breaking cellulose chains into sugar molecules by suitable enzymes. The cellulose present in lignocellulosic biomass is recalcitrant to enzyme systems, largely due to the complex structure of plant cell walls, and generally requires pretreatment to make the cellulose fraction accessible by enzymatic hydrolysis. Chemical and organic solvent pretreatments are common, but such techniques are typically accompanied by significant cost, as well as a host of environmental management and waste treatment issues resulting from, or otherwise related to, use of the pretreatment chemicals. Example chemical pretreatment methods include sulfur dioxide treatment, oxidative delignification, ozonolysis, ammonia fiber explosion (AFEX), treatments with organosolvents and/or ionic liquids, and so forth (see, e.g., Zheng, et al., Overview of biomass pretreatment for cellulosic ethanol production, Int. J. Agric. & Biol. Eng., 2(3), pp 51-68, 2009). Besides material cost and regulatory requirements, like acid hydrolysis, chemical pretreatment methods characteristically introduce residual impurities into the hydrolysis reaction co-products. Moreover, many chemical pretreatment processes modify the lignin present in lignocellulosic biomass, and/or introduce impurities such as sulfur and sulfur compounds, which collectively create complexity and cost in isolation and/or use of such lignin, particularly in liquid fuel products for which low sulfur content is a requirement.
Example non-chemical pretreatment methods for enzymatic hydrolysis include steam explosion and liquid hot water pretreatment, both of which require significant energy costs. Moreover, even in these non-chemical methods, the use of hot water or steam have been known to result in acetic acid formation, which in turn reacts with hemicellulose sugar to form furfural during pretreatment (Harmsen et al., Literature review of physical and chemical pretreatment processes for lignocellulosic biomass, ECN-E-10-013, Energy Research Centre of the Netherlands, 2010).
Mechanical pretreatment, such as milling wood chips to fine wood powder to the extent that the tightly structured cell wall is opened, can increase susceptibility to enzymatic hydrolysis by allowing the enzymes to more readily contact the cellulose (see, e.g., Agarwal, et al., Enzymatic hydrolysis of biomass: Effects of crystallinity, particle size, and lignin removal, Proceedings of the 16th ISWPFC, China Light Industry Press, 2011). However, typical milling techniques often require significant energy cost and/or time to yield a suitable particle size. Moreover, milling is often accompanied by one or more chemical processes, such as to remove pitch or lignin, to more efficiently achieve a desired particle size and/or powder consistency. In particular, delignification of milled biomass prior to enzymatic hydrolysis has been found to make the cellulose in milled biomass more accessible to the enzyme system used, such as by further increasing the surface area of the milled material by forming pores via lignin removal (Id.; see also WO2010077170 of Davidov et al.).